1. Field of the Invention
This invention relates to white balance adjusting devices in color video cameras, color electronic still cameras, or other color image sensing apparatuses.
2. Description of the Related Art
To adjust the white balance of the image in the color image sensing apparatus, there has been known the automatic tracking type white balance adjusting device using a colorimetric sensor in the form independent of the image sensing system.
FIG. 1 is a block diagram illustrating the construction of an example of the prior known apparatus comprising an R sensor 10 for detecting the red color component, a B sensor 12 for detecting the blue color component, logarithmic amplifiers 14 and 16 for logarithmically compressing the outputs of the sensors 10 and 12, a differential circuit 18 for producing an output representing the difference between the outputs of the logarithmic amplifiers 14 and 16, a ripple detecting circuit 20 for determining the amount of ripples contained in the output of the differential circuit 18, a color image sensor 22 for producing R, G and B signals, a B amplifier 24 of gain control type for amplifying the B signal output of the color image sensor 22, an R amplifier 26 of gain control type for amplifying the R signal output of the color image sensor 22, control voltage generating circuits 28 and 30 responsive to the output of the differential circuit 18 and the output of the ripple detecting circuit 20 for producing control voltages C.sub.B and C.sub.R for controlling the gains of the B amplifier 24 and the R amplifier 26 respectively, a signal processing system 32 receptive of the outputs of the G output of the color image sensor 22 and the outputs of the B amplifier 24 and the R amplifier 26 for forming prescribed video signals, and a recording circuit 33.
In the white balance adjusting device of FIG. 1, what are provided in separation from the color image sensor 22 are the R sensor 10 and the B sensor 12 by which the red color component S.sub.R and the blue color component S.sub.B are detected out of an object of wide image angle (corresponding to a white object). The outputs of the sensors 10 and 12 are logarithmically compressed in passing through the logarithmic amplifiers 14 and 16, and then supplied to the differential circuit 18. The output of the differential circuit 18 represents log S.sub.R /S.sub.B. The ripple detecting circuit 20 determines the magnitude of the ripples contained in the signal log S.sub.R /S.sub.B. From the outputs of the differential circuit 18 and the ripple detecting circuit 20, the control voltage generating circuits 28 and 30 produce the control voltages C.sub.B and C.sub.R. By these control voltages C.sub.B and C.sub.R, the gains of the B amplifier 24 and the R amplifier 26 are controlled. Thus, white balance adjustment is effected.
In the conventional example of FIG. 1, because only two colors, namely, R and B, are sensed, for a light having no strong spectral lines in the green wavelength region, such as a fluorescent light, as compared with the sun light or tungsten light, its green component cannot be detected. Therefore, use is made of the ripple detecting circuit 20 for sensing the ripples of the signal due to the flickering of the light from the energized fluorescent lamp, so that the control voltages C.sub.B and C.sub.$ are adjusted in accordance with the amount of ripples determined. In more detail, if the ripple amount is large, the light source is assumed to be the fluorescent lamp or the like, and the levels of the signals representing the red and blue components are increased to suppress its green component.
FIG. 2 in block diagram shows another example of the prior known device, where the same reference numerals have been employed to denote the similar constituent parts to those shown in FIG. 1. The device is provided with a G sensor 11, a logarithmic amplifier 15, differential circuits 34 and 36, and control voltage generating circuits 38 and 40 which are similar in construction to the control voltage generating circuits 28 and 30. In this conventional example, all the primary color components S.sub.R, S.sub.G and S.sub.B of the object of wide image angle are detected by the sensors 10, 11 and 12 and are then logarithmically compressed by the logarithmic amplifiers 14, 15 and 16. The differences of the outputs of the logarithmic amplifiers 14, 15 and 16 are taken by the differential circuits 34 and 36. That is, the output of the differential circuit 36 represents log S.sub.R /S.sub.G and the output of the differential circuit 34 represents log S.sub.B /S.sub.G. Depending on these outputs of the differential circuits 34 and 36, the control voltage generating circuits 38 and 40 produce the control voltages C.sub.B and C.sub.R respectively. The white balance is adjusted by controlling the gains of the amplifiers 24 and 26 in accordance with these control voltages C.sub.B and C.sub.R.
In the conventional example of FIG. 2, because the green component, too, is detected, even if the light source is the fluorescent lamp, the correct white balance can be realized.
But, since, in the conventional example of FIG. 1, there is not always correlation between the ripple amount due to the flickering and the strength of the green component of its fluorescent lamp, even if the level of the green signal is controlled in accordance with the ripple amount, the correct white balance adjustment for every type of fluorescent lamp cannot be obtained. Another drawback is that as the level of the signal changes from other reasons than the flickering of the fluorescent lamp, it is mistaken for the flickering of the fluorescent lamp. This leads to an unduly large suppression of the green component.
The second conventional example of FIG. 2 has no such problems as described above. But, for example, in outdoor photography over a greensward, the result of colorimetry of all the sensors 10, 11 and 12 is influenced by the green color of the foreground and/or background so that the object is judged as if it were under a light source of strong green component. In this case, despite the illumination under the outdoor light, compensation of weakening the green is effected, resulting in a white balance adjustment tinted with magenta.